1. Field of the Invention
This invention relates in general to methods for cleaning coils and header columns in an absorption column, and more particularly, to methods f or cleaning a nitric acid absorption column.
2. Prior Art
In many industrial processes it is desirable to treat a dilute (20%) nitric acid stream to form a more concentrate (typically 59%, 70%, or 92%) nitric acid stream. This is accomplished through the use of an absorption column. Typically, such columns may be up to 200 feet tall and 10-15 feet in diameter. There will be a sparger assembly at the top of the column to introduce the dilute nitric acid, and a sparger assembly at the bottom of the column to introduce nitrous gases. Inside the column will be a series of bubble cap or sieve trays wherein each tray holds multiple rows of coils and within each row are multitudes of three-quarter inch or one and one-quarter inch inside diameter cooling coils. The number of coils per tray will vary depending on the cooling requirements of the column, and may depend on such parameters as feed stock and column design. Typically, there are 4-36 coils per tray, and 24-48 trays in a column. These coils are connected at one end to a water source such as a water cooling tower and enter from outside the column via a cooling tower vertical inlet header having horizontal branches, and are positioned in the trays and then exit back to the outside of the column to a return header, and then finally back to the water cooling tower to complete the water circulation flow. Water is the usual cooling fluid circulated in the coils, although chilled brine may be used, particularly in the upper section of the column.
Because the efficiency of the absorption tower depends upon control of the heat of reaction taking place within the column it is important that one can control the amount of water circulating within the coils. However, several problems occur which can restrict the control of water flowing through the coils. It is not uncommon that wood splinters and other solid particles in the water cooling tower enter the water stream circulating within the coils. When this happens it is possible f or a coil or coils to become partially or completely plugged. In addition the coils may develop leaks which allow nitric acid to enter the water stream and flow back to the water tower. When this occurs it is usual to add a base or alkaline material to the water to reduce the effects of the acid to other parts of the water cooling tower. These chemicals in turn result in the formation of precipitates such as iron that will coagulate and eventually partially or completely plug various coils within the absorption tower.
After a period of time it becomes necessary to repair leaking coils, as well as to clean out the plugged coils. The two most common methods of cleaning out the plugged coils include (i) accessing the coils one at a time from outside of the column and trying to blow air or fluid through the coil under sufficient pressure to force out any material causing a blockage, and (ii) acid treating the coils by pumping acid through all of the coils at one time. Both methods have significant drawbacks. First, is that both require a shutdown of the absorption tower for long periods of time while the repairs and cleaning are being carried out. In an industrial environment this effectively shuts down a plant or major process operations of a plant. Because there are so many coils in parallel (300-800) and because of the many bends in the coils and the distance the entrapped air must flow to be displaced from the coils, high pressure is required to flush acid through the coils with no assurance of opening the plugged coils. Pumping through all the tubes at once results in the necessity of using large pumps, and creates a dangerous operation because of the possibility that connecting hoses, particularly if they are old hoses, may be blown off by the pressure. As a result of feeding so many coils in parallel, it is common that after an attempted acid cleaning, a substantial amount of acidic sludge may accidentally remain in the coils which then may be difficult to remove.
Testing for leaks on stream creates additional problems, not the least of which is that in many cases it is not readily apparent where a particular leaking coil is located, nor where its outlet exits the column. Thus, to plug-off a particular leaking coil requires that one first determine where the particular coil exits. This can be a very time consuming task when the tower is on-stream considering that there may be up to 36 coils per tray, and as many as 48 trays per column.
Yet another problem has been discovered by applicant and that is the water supply manifold headers and hoses and the water return manifold headers and hoses may also become plugged. Unless these headers and hoses are unplugged, water or other cooling liquid can not efficiently circulate through the coils in the absorption column.